Tests for StackerSetUpdater

[zone117x]

The StackerSet fetcher background service needs tests.

It's responsible for populating the db reward_set_signers table. There's some indication that it might get stuck after running for a while.

Here's how it works (which is also an outline of how to implement tests):

1. Listen to the missingStackerSet for cycle N events from the db.
2. Queue a job in memory (using p-queue).
3. During the job execution it fetch the set for cycle N (via stacks-core RPC) and then store it in the db.
4. If the RPC fetch or the db write fails, the job is re-queued to try again after a timeout.